So Many Fiber Questions


The Right Ones Get You the BEST Answers —

Many operators are expanding their efforts in the FTTB arena and need more fiber. Wireless data is increasing demand for fiber backhaul, and 5G deployments will require even more. If your network supports any of these applications, you probably need more fiber.

If you’ve been building FTTx (or anything else for that matter), it can be pretty tempting to keep doing what has worked for you. After all, you probably spent a lot of time working the bugs out of your current network design. However, FTTB and 5G raise a new challenge: there’s usually no way to know where all the access points will be. This is a game changer in terms of network design and product selection. So, it makes sense to take a fresh look at your fiber cabling options.

When you start looking at fiber optic cable, the choices can be daunting. But, if you ask the right questions, it’s much easier to sort through the options. To get started, we’ll describe some key cable design categories. Since some of these designs may be unfamiliar, some photos are also provided in Figure 1. Then, we’ll evaluate these options using some simple, practical questions.

Figure 1.

Loose Tube Cable uses loose fibers in buffer tubes that are S-Z stranded (alternating directions) around a central strength member. Loose Tube cables are now available in smaller form-factors, which have the same performance capabilities as full-sized designs. Reduced Diameter Loose Tube also includes microduct cables. But, they tend to be less robust, and often have less flexible buffer tubes.

Central Tube Ribbon cables employ a single buffer tube. Numerous ribbons are organized into a stack, which is continuously twisted to relieve bending stresses. As fiber counts increase, standard 12-fiber ribbons are joined together into 24-fiber or even 36-fiber ribbons, in order to create a wider, more stable ribbon stack. Since ribbons are located in the middle of the cable, Radial Strength Members (RSMs) are embedded in the jacket.

Stranded/Multi-Tube Ribbon employs several tubes, which are S-Z stranded around a Central Strength Member (CSM). For counts up to 864, these designs typically use only 12-fiber ribbons (not 24 or 36).

Flexible Micro-Module cables are made with a very soft, thin-walled “tube”. The Micro-Module provides organization like a buffer tube. Yet, it’s much smaller and more flexible than even a reduced-diameter buffer tube. Micro-Modules are typically cabled in a structure that is very similar to a Central Tube ribbon cable, with RSMs embedded in the jacket. However, there is usually not a buffer tube, per se.

Retractable Modules are small, hard, and slippery. They typically contain only a few fibers each, and are placed in a cable structure that is similar to a Central Tube ribbon cable. Retractable Modules are very loosely contained by the cable, which allows long lengths (30+ feet) to be retracted at access points.

The Details Matter
Now that we’ve described the cables, we’ll review each category in light of 7 key questions related to the kind of network you plan to build. The results will also be summarized in Figure 2, a table.

Figure 2.

Question #1
Do I need high fiber density?
Higher fiber density can be helpful when space is limited. It’s important to realize that designs known for high fiber density (e.g., ribbon) may not have the highest (best) density at all fiber counts.

Fiber Density also factors into minimum bend radius. The minimum bend radius of a cable is determined by cable outside diameter (OD). So, smaller cables can be coiled in smaller loops and handholes.

Question #2
Will the cable need to be coiled frequently?
This is important if you’re going to have a lot of storage coils — especially if they’re in handholes. Struggling to fit a coil in a tight handhole is time-consuming, and is an easy way to end up with cable damage. If you use bigger handholes, it will drive up the cost of materials and installation.

For frequent coiling, your best choice is Reduced Diameter Loose Tube, followed by Standard (full-size) Loose-Tube and Multi-Tube Ribbon cables.

Cables with RSMs will bend only in one plane. This makes them much more awkward to coil than cables with CSMs. This is even more of an issue for higher fiber counts, since this difficulty is magnified by larger cable diameters.

Central Tube Ribbon and Flexible Micro-Module cables are all made with RSMs. Micro Module cables are also made with RSMs. Fortunately, as we’ll see later on, coils aren’t needed to facilitate access of Micro-Module cables.

Question #3
How frequently will I access the cable?
The range of 5G is expected to be quite short, which will require many access points. So, it makes sense to consider the time required to open the cable and prep it for a closure.

For frequent access, Retractable Module cables are best, followed by Loose Tube (all sizes) and Multi-Tube Ribbon cables.

In general, cables with RSMs take slightly longer to open because their jackets are thicker. The increased thickness is necessary to accommodate the RSMs, which are embedded in the jacket. Another option is to shave the jacket over the RSMs. This can be done quickly by experienced installers, but requires significant physical strength.

Retractable Module cables are an exception to this trend. The traditional access steps are bypassed by opening a “window” in the jacket with a special tool. Because the RSMs aren’t cut, simplified closures can be attached directly to the jacket.

Question #4
Do I need a cable that is convenient for taut-sheath access?
Taut-sheath access occurs when a cable must be opened in a location with no slack. Taut-sheath access can be difficult with some cables, while other cables are designed specifically for it. You can avoid taut-sheath access by installing slack coils up front if you know where they will be needed. This will be tough to anticipate, since 5G hasn’t been standardized. And, as we saw earlier, coils also cost time and material.

The best choice for taut-sheath access is Retractable Module cable, followed by cables with S-Z stranded tubes (Multi-Tube Ribbon and all sizes of Loose Tube).

Retractable Module cables were specifically created to simplify taut-sheath access. Long lengths (30+ feet) of fiber can be quickly and easily extracted.

S-Z stranded tubes reverse direction periodically. The reversal point provides extra tube length even when there is no cable slack. Though small, this extra length can make taut-sheath access much easier. This benefit applies to both stranded loose tube and stranded ribbon designs.

Because they are not stranded, Central-Tube Ribbon cables lack this benefit. Moreover, the continuous twist of the ribbon stack makes it harder to access a ribbon from the middle of a stack — especially if live traffic is present on the other ribbons. Flexible Module cables are similar, although it’s easier to get to individual fibers.

Question #5
How will I manage “Express” fibers?
In applications with lots of access points, it’s common to “express” the untapped fibers, rather than splice them. If you’ll be doing this a lot, it should be quick and easy to identify, separate, and store the express fibers. If you’ll be adding access points gradually, you also need a design that allows this to be done without disrupting live traffic.

Express Fibers are easiest to manage in Retractable Module cables and Reduced Diameter Loose Tube cables, followed by Standard Loose Tube and Multi-Tube Ribbon.

Designs that subdivide fibers into smaller groups can limit the number of fibers which must be exposed. This is a plus when dealing with live traffic. Just make sure the elements being expressed are flexible and easily stored.

Most standard loose tube designs provide good tube flexibility. Because their smaller, reduced diameter loose tubes are even more flexible, and will take up less storage space. This permits higher counts and/or smaller closures. Flexible Module cables take this trend to the extreme.

Stranded Tube ribbon cables offer similar benefits over Central Tube ribbon cables of the same count when produced with flexible tube materials. (Check with your cabler on this point.)

By definition, Central Tube ribbon cables place all fibers/ribbons in a common tube. Individual ribbons are printed with unique identifiers and can be “expressed”. But, transport tubing should be installed to provide mechanical protection. This adds time and cost, and can be risky where live traffic is present.

A key benefit of Retractable Module cables is that Express Fibers are left undisturbed in the cable. However, care must be taken while probing through the modules to identify the module which is targeted for retraction.

Question #6
Do I need to access small numbers of fibers?
In many cases, an access point will require fewer than 12 fibers. This requires further consideration, since it will require the sub-division of tubes or ribbons. If done frequently, it needs to be simple, quick, and safe for live traffic.

Flexible Module, Retractable Module, and Loose Tube cables (all sizes) are all good options when accessing small numbers of fibers. Loose Tube is best where live traffic is present.

Mid-span tube slitters are available for loose-tube cables. These are quick and fairly easy to use. Just be sure to use approved tools or you’ll risk microscopic fiber damage.

Individual fibers can be accessed within a ribbon. But, it’s an added step that has to be done carefully, and is likely to disrupt live traffic. You should consider alternatives to ribbon if you will usually access less than 12 fibers at a time.

Flexible Modules can be removed by hand without any tools. This can be done for both end-access and mid-span. This process does apply pressure to the fibers. So, it may be disruptive to live traffic.

Retractable Module cables typically have very few fibers per module to start with, which makes sub-division less common. However, mid-span sub-division is not practical.

Question #7
How often will I do butt-splices?
There’s no doubt that mass-fusion splicing is more efficient than single fiber splicing. If you frequently butt-splice (or restore) high-count cables, ribbon is your best option.

Ribbon is not a big differentiator in locations where you are only splicing a few fibers. In fact, it’s no help at all when you need to tap 3 or 4 fibers, since you’ll have to break the ribbon open before splicing.

Make sure you take an honest look at how often you’ll enjoy the benefits relative to the potential disadvantages of ribbon.

(The discussion above is summarized in Figure 2.)

As you can see, there’s no “one size fits all” solution. The best cable choice will depend a lot on your network plans. You may also find that the best choice varies by network segment or fiber count, which is why many operators use a mixture of cable types.

It’s tough to cover every detail in a single article. If you’re still unsure, don’t worry — reach out to a trusted cabler. We’re here to help you find the best product for your application




About Author

Jon Fitz is Director of Product Management for Prysmian Group. He has worked in the telecom industry for more than 27 years in a variety of engineering and commercial roles. For more information, please email or visit Follow Prysmian Group on Twitter: @PrysmianGroup. Follow Jon on Twitter: @JonFitz7.

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